An emerging new technology based on genetically engineered viral vectors that can insert genes into the cells of living organisms may play a significant role in treating disorders of the central nervous system (CNS). Since the CNS is highly specialized, disease processes typically affect focal regions of the brain or spinal cord. Preventive or treatment strategies will need to be targeted to the diseased regions, without affecting other areas. Administration of therapeutic genes specifically to the diseased regions of brain may be more beneficial than current treatment strategies, which are largely based on systemic administration of small molecules or proteins. Parkinson's disease (PD) is a good example of a focal brain disorder in which slow degeneration of dopamine-producing neurons, mostly in the nigrostriatal pathway, results in neurological symptoms. Since dopamine cannot cross the blood-brain-barrier (BBB), the current treatment for PD is a replacement strategy in which the dopamine precursor L-Dopa, which can pass the BBB, is given orally several times a day. The biosynthetic pathway for dopamine contains two critical enzymes, tyrosine hydroxylase (TH) which converts the amino acid tyrosine to dopa, and aromatic-amino-dopa-decarboxylase (AADC) which decarboxylates dopa to dopamine. Administration of L-dopa bypasses the TH step, and L-Dopa is converted to dopamine by AADC. Unfortunately, the effectiveness of this treatment strategy is limited because nigrostriatal AADC levels decrease with disease progression, thereby preventing the conversion of L-Dopa to dopamine in sufficient quantities.We have tested a new treatment approach that combines gene therapy with the AADC gene and a pro-drug (L-dopa) to produce dopamine in brain. We performed experiments in parkinsonian monkeys where AADC was administered via an adeno-associated viral vector (AAV) by convection-enhanced delivery (CED) unilaterally into the striatum. We showed that AAV can be safely distributed over an entire anatomical target region of a monkey brain. The gene transfer that we obtained was extremely efficient (almost 20 million striatal neurons expressed the transgene). Our study demonstrated that positron emission tomography (PET) with the AADC tracer 6-[18F]fluoro-L-m-tyrosine (FMT) can be used to monitor gene therapy in vivo. FMT activity was restored back to normal levels in parkinsonian monkeys following AAV-AADC gene transfer and it correlated well post-mortem analysis. This gene therapy approach restored AADC activity to normal levels in the treated animals, but not in control animals that were treated with AAV-LacZ. AADC-expressing striatal neurons were also able to store dopamine, and may therefore provide a buffer for unmetabolized L-Dopa. This approach to treating PD may reduce the need for L-Dopa/carbidopa, providing a better